ospf faq

Q. Why are loopbacks advertised as /32 host routes in OSPF?

A. Loopbacks are considered host routes in OSPF, and they are advertised as /32. For more

information, refer to section 9.1 of RFC 2328. In Cisco IOS Software Releases 11.3T and

12.0, if the ip ospf network point−to−point command is configured under loopbacks, OSPF

advertises the loopback subnet as the actual subnet configured on loopbacks. ISDN dialer

interface advertises /32 subnet instead of its configured subnet mask. This is an expected

behavior if ip ospf network point−to−multipoint is configured.

For example, consider two routers (R1 and R2) connected via FastEthernet interface. R1 has

the loopback configured with the ip ospf network point−to−point command and advertises

the loopback in OSPF.

interface Loopback0

ip address 1.1.1.1 255.255.255.0

ip ospf network point−to−point

When checked in router R2 with the show ip route ospf command, the route 1.1.1.1 is seen

as:

!..output truncated

1.0.0.0/24 is subnetted, 1 subnets

O 1.1.1.0 [110/11] via 10.1.1.1, 00:00:02, FastEthernet0/0

However, when the ip ospf network point−to−point command is removed from R1 to 0

interface, the route 1.1.1.1 on R2 is seen as:

1.0.0.0/32 is subnetted, 1 subnets

O 1.1.1.1 [110/11] via 10.1.1.1, 00:00:01, FastEthernet0/0

Q. How do I change the reference bandwidth in OSPF?

A. You can change the reference bandwidth in Cisco IOS Software Release 11.2 and later

using the ospf auto−cost reference−bandwidth command under router ospf. By default,

reference bandwidth is 100 Mbps. The ospf link−cost is a 16−bit number. Therefore, the

maximum value supported is 65,535.

Q. How does OSPF calculate its metric or cost?

A. OSPF uses a reference bandwidth of 100 Mbps for cost calculation. The formula to

calculate the cost is reference bandwidth divided by interface bandwidth. For example, in the

case of Ethernet, it is 100 Mbps / 10 Mbps = 10.

Note: If ip ospf cost cost is used on the interface, it overrides this formulated cost. For more

information, refer to OSPF Cost.

Q. What algorithm is used by OSPF if equal cost routes exist?

A. If equal cost routes exist, OSPF uses CEF load balancing. For more information, refer to

Troubleshooting Load Balancing Over Parallel Links Using Cisco Express Forwarding.

Q. Are OSPF routing protocol exchanges authenticated?

A. Yes, OSPF can authenticate all packets exchanged between neighbors. Authentication may

be through simple passwords or through MD5 cryptographic checksums. To configure simple

password authentication for an area, use the command ip ospf authentication−key to assign

a password of up to eight octets to each interface attached to the area. Then, issue the area x

authentication command to the OSPF router configuration to enable authentication. (In the

command, x is the area number.)

Cisco IOS Software Release 12.x also supports the enabling of authentication on a

per−interface basis. If you want to enable authentication on some interfaces only, or if you

want different authentication methods on different interfaces that belong to the same area, use

the ip ospf authentication interface mode command.

Q. What is the link−state retransmit interval, and what is the command to

set it?

A. OSPF must send an acknowledgment of each newly received link−state advertisement (LSA).

It does this by sending LSA packets. LSAs are retransmitted until they are acknowledged.

The link−state retransmit interval defines the time between retransmissions. You can use the

command ip ospf retransmit−interval to set the retransmit interval. The default value is 5

seconds.

Q. What is the purpose of the variable IP−OSPF−Transmit−Delay?

A. This variable adds a specified time to the age field of an update. If the delay is not added

before transmission over a link, the time in which the link−state advertisement (LSA)

propagates over the link is not considered. The default value is 1 second. This parameter has

more significance on very low−speed links.

Q. Is it true that only the static option of the virtual link in OSPF allows

discontiguous networks, regardless of the mask propagation

properties?

A. No, virtual links in OSPF maintain connectivity to the backbone from nonbackbone areas,

but they are unnecessary for discontiguous addressing. OSPF provides support for

discontiguous networks because every area has a collection of networks, and OSPF attaches a

mask to each advertisement.

Q. Are the multicast IP addresses mapped to MAC−level multicast

addresses?

A. OSPF sends all advertisements using multicast addressing. Except for Token Ring, the

multicast IP addresses are mapped to MAC−level multicast addresses. Cisco maps Token

Ring to MAC−level broadcast addresses.

Q. Does the Cisco OSPF implementation support IP TOS−based routing?

A. Cisco OSPF only supports TOS 0. This means that routers route all packets on the TOS 0

path, eliminating the need to calculate nonzero TOS paths.

Q. Does the offset−list subcommand work for OSPF?

A. The offset−list command does not work for OSPF. It is used for distance vector protocols

such as Interior Gateway Routing Protocol (IGRP), Routing Information Protocol (RIP), and

RIP version 2.

Q. Can an OSPF default be originated into the system based on external

information on a router that does not itself have a default?

A. OSPF generates a default only if it is configured using the command default−information

originate and if there is a default network in the box from a different process. The default

route in OSPF is 0.0.0.0. If you want an OSPF−enabled router to generate a default route even

if it does not have a default route itself, use the command default−information originate

always.

Q. Can I use the distribute−list in/out command with OSPF to filter

routes?

A. The distribute−list commands are supported in OSPF but work differently than

distance−vector routing protocols such as Routing Information Protocol (RIP) and Enhanced

Interior Gateway Routing Protocol (EIGRP). OSPF routes cannot be filtered from entering

the OSPF database. The distribute−list in command only filters routes from entering the

routing table; it does not prevent link−state packets from being propagated. Therefore, this

command does not help conserve router memory, and it does not prohibit a router from

propagating filtered routes to other routers.

Caution: Use of the distribute−list in command in OSPF may lead to routing loops in

the network if not implemented carefully.

The command distribute−list out works only on the routes being redistributed by the

Autonomous System Boundary Routers (ASBRs) into OSPF. It can be applied to external

type 2 and external type 1 routes, but not to intra−area and interarea routes.

Refer to configuration example of distribute−list in OSPF,

Q. How can I give preference to OSPF interarea routes over intra−area

routes?

A. According to section 11 of RFC 2328, the order of preference for OSPF routes is:

•intra−area routes, O

•interarea routes, O IA

•external routes type 1, O E1

•external routes type 2, O E2

This rule of preference cannot be changed. However, it applies only to a single OSPF

process. If a router is running more than one OSPF process, route comparison occurs. With

route comparison, the metrics and administrative distances (if they have been changed) of the

OSPF processes are compared. Route types are disregarded when routes supplied by two

different OSPF processes are compared.

Q. Do I need to manually set up adjacencies for routers on the Switched

Multimegabit Data Service (SMDS) cloud with the OSPF neighbor

subcommand?

A. In Cisco IOS Software releases earlier than Cisco IOS Software Release 10.0, the

neighbor command was required to establish adjacencies over nonbroadcast multiaccess

(NBMA) networks (such as Frame Relay, X.25, and SMDS). With Cisco IOS Software

Release 10.0 and later, you can use the ip ospf network broadcast command to define the

network as a broadcast network, eliminating the need for the neighbor command. If you are

not using a fully meshed SMDS cloud, you must use the ip ospf network

point−to−multipoint command.

Q. When routes are redistributed between OSPF processes, are all

shortest path first algorithm (SPF) metrics preserved or is the default

metric value used?

A. The SPF metrics are preserved. The redistribution between them is like redistribution

between any two IP routing processes.

Q. How does Cisco accommodate OSPF routing on partial−mesh Frame

Relay networks?

A. You can configure OSPF to understand whether it should attempt to use multicast facilities

on a multi−access interface. Also, if multicast is available, OSPF uses it for its normal

multicasts.

Cisco IOS Software Release 10.0 includes a feature called subinterfaces. You can use

subinterfaces with Frame Relay to tie together a set of virtual circuits (VCs) to form a virtual

interface, which acts as a single IP subnet. All systems within the subnet should be fully

meshed. With Cisco IOS Software Releases 10.3, 11.0 and later, the ip ospf

point−to−multipoint command is also available.

Q. Which address−wild−mask pair should I use for assigning an

unnumbered interface to an area?

A. When an unnumbered interface is configured, it references another interface on the router.

When enabling OSPF on the unnumbered interface, use the address−wild−mask pair of

interfaces to which the unnumbered interface is pointing.

Q. Can I have one numbered side and leave the other side unnumbered

in OSPF?

A. No, OSPF does not work if you have one side numbered and the other side unnumbered.

This creates a discrepancy in the OSPF database that prevents routes from being installed in

the routing table.

Q. Why do I receive the "cannot allocate router id" error message when I

configure Router OSPF One?

A. OSPF picks up the highest IP address as a router ID. If there are no interfaces in up/up

mode with an IP address, it returns this error message. To correct the problem, configure a

loopback interface.

Q. Why do I receive the "unknown routing protocol" error message when

I configure Router OSPF One?

A. Your software may not support OSPF. This error message occurs most frequently with the

Cisco 1600 series routers. If you are using a 1600 router, you need a Plus image to run OSPF.

Q. What do the states DR, BDR, and DROTHER mean in show ip ospf

interface command output?

A. DR means designated router. BDR means backup designated router. DROTHER indicates a

router that is neither the DR or the BDR. The DR generates a Network Link−State

Advertisement, which lists all the routers on that network.

Q. When I issue the show ip ospf neighbor command, why do I only see

FULL/DR and FULL/BDR, with all other neighbors showing

2−WAY/DROTHER?

A. To reduce the amount of flooding on broadcast media, such as Ethernet, FDDI, and Token

Ring, the router becomes full with only designated router (DR) and backup designated router

(BDR), and it shows 2−WAY for all other routers.

Q. Why do I not see OSPF neighbors as FULL/DR or FULL/BDR on my

serial link?

A. This is normal. On point−to−point and point−to−multipoint networks, there are no

designated routers (DRs) or backup designated routers (BDRs).

Q. Do I need any special commands to run OSPF over BRI/PRI links?

A. In addition to the normal OSPF configuration commands, you should use the dialer map

command. When using the dialer map command, use the broadcast keyword to indicate that

broadcasts should be forwarded to the protocol address.

Q. Do I need any special commands to run OSPF over asynchronous

links?

A. In addition to the normal OSPF configuration commands, you should use the async

default routing command on the asynchronous interface. This command enables the router to

pass routing updates to other routers over the asynchronous interface. Also, when using the

dialer map command, use the broadcast keyword to indicate that broadcasts should be

forwarded to the protocol address.

Q. Which Cisco IOS Software release began support for per−interface

authentication type in OSPF?

A. Per−interface authentication type, as described in RFC 2178, was added in Cisco IOS

Q. Can I control the P−bit when importing external routes into a

not−so−stubby area (NSSA)?

A. When external routing information is imported into an NSSA in a type 7 link−state

advertisement (LSA), the type 7 LSA has only area flooding scope. To further distribute the

external information, type 7 LSAs are translated into type 5 LSAs at the NSSA border. The

P−bit in the type 7 LSA Options field indicates whether the type 7 LSA should be translated.

Only those LSAs with the P−bit set are translated. When you redistribute information into the

NSSA, the P−bit is automatically set. A possible workaround applies when the Autonomous

System Boundary Router (ASBR) is also an Area Border Router (ABR). The NSSA ASBR

can then summarize with the not−advertise keyword, which results in not advertising the

translated type 7 LSAs.

Q. Why are OSPF show commands responding so slowly?

A. You may experience a slow response when issuing OSPF show commands, but not with

other commands. The most common reason for this delay is that you have the ip ospf

name−lookup configuration command configured on the router. This command causes the

router to look up the device Domain Name System (DNS) names for all OSPF show

commands, making it easier to identify devices but resulting in a slowed response time for

the commands. If you are experiencing slow response on commands other than just OSPF

show commands, you may want to start looking at other possible causes, such as the CPU

utilization.

Q. What does the clear ip ospf redistribution command do?

A. The clear ip ospf redistribution command flushes all the type 5 and type 7 link−state

advertisements (LSAs) and scans the routing table for the redistributed routes. This causes a

partial shortest path first algorithm (SPF) in all the routers on the network that receive the

flushed/renewed LSAs. When the expected redistributed route is not in OSPF, this command

may help to renew the LSA and get the route into OSPF.

Q. Does OSPF form adjacencies with neighbors that are not on the same

subnet?

A. The only time that OSPF forms adjacencies between neighbors that are not on the same

subnet is when the neighbors are connected through point−to−point links. This may be

desired when using the ip unnumbered command, but in all other cases, the neighbors must

be on the same subnet.

Q. How often does OSPF send out link−state advertisements (LSAs)?

A. OSPF sends out its self−originated LSAs when the LSA age reaches the link−state refresh

time, which is 1800 seconds. For more information, refer to Link−State Advertisements.

Q. How do I stop individual interfaces from developing adjacencies in an

OSPF network?

A. To stop routers from becoming OSPF neighbors on a particular interface, issue the

passive−interface command at the interface.

In Internet service provider (ISP) and large enterprise networks, many of the distribution

routers have more than 200 interfaces. Configuring passive−interface on each of the 200

interfaces can be difficult. The solution in such situations is to configure all the interfaces as

passive by default using a single passive−interface default command. Then, configure

individual interfaces where adjacencies are desired using the no passive−interface command.

For more information, refer to Default Passive Interface Feature.

There are some known problems with the passive−interface default command. Workarounds

are listed in Cisco bug ID CSCdr09263 ( registered customers only).

Q. When I have two type 5 link−state advertisements (LSAs) for the same

external network in the OSPF database, which path should be installed

in the IP routing table?

A. When you have two type 5 LSAs for the same external network in the OSPF database,

prefer the external LSA that has the shortest path to the Autonomous System Boundary

Router (ASBR) and install that into the IP routing table. Use the show ip ospf

border−routers command to check the cost to the ASBR.

Q. Why is it that my Cisco 1600 router does not recognize the OSPF

protocol?

A. Cisco 1600 routers require the Plus feature set image of Cisco IOS Software to run OSPF.

Refer to Table 3: Cisco 1600 Series Routers Feature Sets in the Release Notes for Cisco IOS

Release 11.2(11) Software Feature Packs for Cisco 1600 Series Routers for more information.

Q. Why is it that my Cisco 800 router does not run OSPF?

A. Cisco 800 routers do not support OSPF. However, they do support Routing Information

Protocol (RIP) and Enhanced Interior Gateway Routing Protocol (EIGRP). You can use the

Software Advisor ( registered customers only) tool for more information on feature support.

Q. Should I use the same process number while configuring OSPF on

multiple routers within the same network?

A. OSPF, unlike Border Gateway Protocol (BGP) or Enhanced Interior Gateway Routing

Protocol (EIGRP), does not check the process number (or autonomous system number) when

adjacencies are formed between neighboring routers and routing information is exchanged.

The only case in which the OSPF process number is taken into account is when OSPF is used

as the routing protocol on a Provider Edge to Customer Edge (PE−CE) link in a Multiprotocol

Label Switching (MPLS) VPN. PE routers mark OSPF routes with the domain attribute

derived from the OSPF process number to indicate whether the route originated within the

same OSPF domain or from outside it. If the OSPF process numbering is inconsistent on PE

routers in the MPLS VPN, the domain−id OSPF mode command should be used to mark that

the OSPF processes with different numbers belong to the same OSPF domain.

This means that, in many practical cases, you can use different autonomous system numbers

for the same OSPF domain in your network. However, it is best to use consistent

OSPF−process numbering as much as possible. This consistency simplifies network

maintenance and complies with the network designer intention to keep routers in the same

OSPF domain.

Q. I have a router that runs Cisco Express Forwarding (CEF) and OSPF,

who does load−balancing when there are multiple links to a destination?

A. CEF works by performing the switching of the packet based on the routing table which is

populated by the routing protocols such as OSPF. CEF does the load−balancing once the

routing protocol table has been calculated. For more details on load balancing, refer to How

does load−balancing work?

Q. How does OSPF use two Multilink paths to transfer packets?

A. OSPF uses the metric aCost, which is related to the bandwidth. If there are equal cost paths

(the same bandwidth on both multilinks), OSPF installs both routes in the routing table. The

routing table tries to use both links equally, regardless of the interface utilization. If one of the

links in the first multilink fails, OSPF does not send all the traffic down the second multilink.

If the first multilink peaks 100%, OSPF does not send any traffic down the second multilink

because OSPF tries to use both links equally, regardless of the interface utilization. The

second is used fully only when the first multilink goes down.

Q. How can you detect the topological changes rapidly?

A. In order to have a rapid fault detection of topology changes, the hello timer value needs to

be set to 1 second. The hold timer value, which is four times that of the hello timer, also

needs to be configured. There is a possibility of more routing traffic if the hello and hold

timer values are reduced from their default values.

Q. Does the 3825 Series Router support the OSPF Stub feature?

A. Yes, the 3800 Series Router that runs Advanced IPServices image supports the OSPF Stub

feature.

Q. What does the error message %OSPF−4−FLOOD_WAR: Process

process−id re−originates LSA ID ip address type−2 adv−rtr ip address in

area area id means?

A. The error message is due to the some router that is flushing the network LSA because the

network LSA received by the router whose LSA ID conflicts with the IP address of one of the

router's interfaces and flushes the LSA out of the network. For OSPF to function correctly the

IP addresses of transit networks must be unique. If it is not unique the conflicting routers

reports this error message. In the error message the router with the OSPF router ID reported

as adv−rtr reports this message.

Q. Can we have OSPF run over a GRE tunnel?

A. Yes, refer to Configuring a GRE Tunnel over IPSec with OSPF.

Q. Is there a way to manipulate and prefer the Type 3 LSAs to originate

from two different areas sent to the non−backbone area?

A. Type 3 LSA is originated by the Area Border Router (ABR) as a summary route.

Manipulating the summary route is not possible in an ABR router.

Q. Is there a drop/flap of an OSPF neighborship when changing an OSPF

area type from nssa no−summary to nssa?

A. When the NSSA ABR is configured to move from nssa no−summary to nssa, the OSPF

neighborship does not flap.

Q. In the %OSPF−5−ADJCHG: Process ID, Nbr [ip−address] on

Port−channel31 from FULL to EXSTART, SeqNumberMismatch

error message, what does SeqNumberMismatch signify?

A. The OSPF neighbor was changed state from FULL to EXSTART because of the receipt of

a Database Description (DBD) packet from the neighbor with an unexpected sequence

number.

SeqNumberMismatch means that a DBD packet during OSPF neighborship negotiation

has been received that either:

•has an unexpected DBD sequence number

•unexpectedly has the Init bit set

has an Options field differing from the last Options field received in a Database

Description packet.

•

Q. What is the maximum number of OSPF processes (VRF aware) on

7600/6500 platforms?

A. Cisco IOS has a limit of 32 routing processes. Two of these are saved for static and

directly connected routes. The Cisco 7600 router supports 28 OSPF processes per VRF.

Q. How does ISPF impact or improve the OSPF network?

A. Incremental SPF is more efficient than the full SPF algorithm, thereby allowing OSPF to

converge faster on a new routing topology in reaction to a network event. The incremental

SPF is designed in such a way that it only updates the affected nodes without rebuilding the

whole tree. This results in a faster convergence and saves CPU cycles because the unaffected

nodes do not need to be processed. Concerning the best practice ISPF would make more of a

difference for a large OSPF domain.

Incremental SPF provides greater improvements in convergence time for networks with a

high number of nodes and links. Incremental SPF also provides a significant advantage when

the changes in the network topology are further away from the root of the SPT; for example,

the larger the network the more significant the impact. A segment of 400−1000 nodes should

see improvements. However, it might be hard to verify in a deployed production network

without some kind of facility or tool to measure the end−to−end delay. For more information,

refer to OSPF Incremental SPF.

Q. Is there a way to compare Cisco NX−OS/IOS OSPF commands?

A. Yes, refer to Cisco NX−OS/IOS OSPF Comparison.